Mongolian spots -- more properly called slate grey nevi -- are very common birthmarks. They are flat and bluish-gray, almost bruise-like. You might be concerned if such a mark show up on your baby, but there's really no reason to worry. Mongolian spots don't hurt, and they won't get any worse as a child gets older. In fact, they often fade over time. By the time your child is in grade school, there's a good chance that you wont be able to see the mark at all. And since the marks most often show up in out-of-the-way places ---usually the lower back but also possibly the buttocks, arms, and legs -- they aren't likely to affect a child's appearance or self-esteem. You may need to explain them to some doctors or clinicians, though, especially if they are not familiar with them.
Nobody knows what causes mongolian spots. Any baby can be born with them, but they are most common in babies with dark skin, including babies of Asian, African, or Native American heritage.
Cleveland Clinic. Birthmarks. http://my.clevelandclinic.org/disorders/birthmark/hic_birthmarks.aspx
Mayo Clinic. Birthmarks slide show. http://www.mayoclinic.com/health/birthmarks/SN00033
Nemours Foundation. Birthmarks. http://kidshealth.org/PageManager.jsp?dn=KidsHealth& lic=1& ps=107& cat_id=189& article_set=35902